This invention relates to the heat treatment of workpieces, and, more particularly, to plasma processing of workpieces for reactive or nonreactive heat treatment.
Many materials such as metals and ceramics are heat treated during manufacturing operations. The heat treatment is typically accomplished by heating a workpiece to elevated temperature and holding the workpiece at that temperature for a period of time. The heat treatment may involve heating to a single temperature, or more complex processing such as a series of heating and cooling steps. The heat treatment may also involve one large workpiece, or many smaller workpieces.
The heat treatment of the workpiece(s) is accomplished for any of a variety of reasons. The microstructure and thence properties of many metallic alloys can be varied by heat treatment. Ceramic pieces processed from powders can be sintered to increase their densities and thence improve their strength properties. These types of processing can be accomplished in vacuum or an inert atmosphere. The heat treatment can also involve processing in a reactive gas such as a source of nitrogen or carbon. Such processing introduces an element from the reactive gas into the surface of the workpiece(s) so as to modify its surface properties but not its interior properties. Many combinations of these treatments, and many other treatments, are possible.
The apparatus used to perform a heat treatment must heat the workpiece(s) and, in the situations of interest here, also must control the atmosphere surrounding the workpiece(s) as the heat treating is performed. One well-established approach involves placing the workpiece(s) into a chamber in which the atmosphere can be controlled, and surrounding the chamber with a heater such as a resistance or inductive heater. Such processing apparatus has been available for many years.
More recently, plasma processing of workpiece(s) has been developed to accomplish heat treating more quickly and efficiently than alternative approaches. A plasma composed of equal numbers of electrons and ions is produced and contacted to the workpiece(s). Each workpiece is heated by the energy transferred to it from the electrons, the ions, or the electrons and ions together that are in the plasma. The heat treatment can be conducted in an inert-gas plasma or in a reactive-gas plasma, resulting in the deposition of an ionic species at the surface of each workpiece that alters its surface properties. Cooling may be accomplished by forced or non-forced quenching.
The various methods available for plasma processing are operable, but have drawbacks or disadvantages. Some lack precise controllability or are relatively inefficient. Others produce a plasma that cannot be tailored to uniformly or controllably non-uniformly, as desired, envelop the workpiece(s), resulting in uneven processing of the workpiece(s). Still others do not allow for independent control of plasma production and heat treatment, or produce a high density plasma. Others operate at too high a gas pressure to allow for rapid heat treatment.
There is a need for a plasma processing method and apparatus that can be used to nonreactively or reactively heat treat workpiece(s) and overcomes these disadvantages or limitations. The present invention fulfills these needs, and further provides related advantages.